Two Distinct Precursors of Hematopoietic Stem Cells and Endothelial Progenitors Characterized by the PCLP1 Expression.

Blood cells and endothelia are believed to arise from their common progenitor hemangioblast. However, it still remains unknown how these lineages develop. Here we report the existence of two distinct precursors for hematopoietic stem cells (HSCs) and endothelial progenitors in murine fetal liver (FL). Podocalyxin-like protein 1 (PCLP1) is a member of the sialomucin family and was shown to be expressed in hemangioblasts in the aorta-gonad-mesonephros region in murine embryo. To further analyze the fates of hematopoietic/endothelial cells, we focused on embryonic day 14.5 (E14.5) FL, since it is a major hematopoietic organ during embryonic period. Based on the PCLP1 expression levels, E14.5 FL cells could be fractionated into four distinct populations. In vitro colony-forming assay and in vivo transplantation analysis revealed that lineage-committed progenitors with colony-forming activities and long-term repopulating hematopoietic stem cells (LTR-HSCs) were in PCLP1neg cells. PCLP1dull cells contained erythroid lineage-committed cells. Interestingly, while PCLP1med cells lacked colony-forming activities, they showed LTR-HSC activity in vivo. To further characterize these cell populations, we cultured them with OP9 stromal cells, since OP9 cells have been used to induce hematopoietic and endothelial lineages from embryonic stem cells. In co-culture with OP9 cells, PCLP1neg cells immediately generated blood cells with colony-forming activity but lacking in vivo hematopoietic activity, indicating that OP9 cells failed to support hematopoietic progenitor/HSCs. However, PCLP1med generated colony-forming hematopoietic progenitors with LTR-HSC activities in the presence of OP9 cells. These results indicated that PCLP1med cells contained stromal cell-dependent immature precursors for HSCs. PCLP1high cells did not express the hematopoietic markers or endothelial cell markers such as PECAM1 and VE-cadherin. However, they formed endothelia-like cell colonies which were highly proliferative and serially transferable in OP9 co-culture. Interestingly, the addition of vascular endothelial growth factor (VEGF) to the culture strongly induced the expression of PECAM1 and VE-cadherin in these colonies. PCLP1high cells contributed to PECAM1+ endothelium in several organs in vivo when transplanted to conditioned neonatal liver. Therefore, PCLP1high cells contained immature precursors for endothelial progenitors. These results indicate that PCLP1 expression levels distinguish previously unrecognized early precursors for HSCs and endothelial progenitors, which are distinct from hemangioblasts.